Should loss of potential be enabled in protective relays? A while back someone told me, and I'm now thinking about it (a lot)- that for a remote end fault (especially on a weak system) the positive sequence voltage at the relay can dip to near zero without an increase in positive or zero sequence current. This would certainly initiate LOP.
The question is, should I set a 120 cycle time delay for LOP? Or just not enable it? What are the consequences of having step distance elements running with a blown fuse or total loss of all 3 phase inputs?
LOP is required in coal mining regulations. It is typically implemented as an undervoltage (UV) relay that is simply a relay held under power like a shunt trip except that it works in reverse...as long as it is powered up it releases the trip latch. If it loses power, a spring pulls the trip latch causing the breaker to open.
Three reasons to consider it. First is do you have trip power available on LOP? It’s one thing on batteries where LOP might just involve monitoring DC voltage. It’s quite another on a capacitive trip device where you have a about a minute before losing power, or about 0.1 seconds if there is no backup power for the relay itself,
Now this might seem silly. After all why not just leave everything alone and just come back up under power just the way things are? The first consideration is what if we lose trip power and nobody notices that protection has failed? It’s easy to do if it fails silently but hard to ignore if it trips. Second consideration is what if we restore power into a fault? It takes some time, usually a second or so, to reboot the relay. Some have a hardware-based trip function set to say 10x plug rating to trip so that it can trip even during reboots but others cannot do this. Finally although this is more of an overall issue some equipment such as motors can be damaged or operate erratically from voltage unbalance or low voltage conditions and it may be better/easier/safer to trip on low voltage as opposed to tripping on other consequences like motor current unbalance or overload.
The alternative is that every time you lose power (LOP) someone has to close in all the affected breakers.
